Jamming signal producing system in CATV

ABSTRACT

A system for producing a jamming signal to be inserted in a band of CATV has a voltage producing circuit for producing a control voltage corresponding to the band, and a first voltage-controlled oscillator for producing a first jamming signal having a frequency dependent on the control voltage, and capacitors for storing the control voltage. A second jamming signal is produced by a second jamming signal producing circuit from the charged stored control voltage. A CPU is provided for controlling the production of the first and second jamming signals.

BACKGROUND OF THE INVENTION

The present invention relates to a system provided in a CATV (cabletelevision) for producing a jamming signal, thereby preventingnonsubscribers from poaching pay television programs.

A subscriber who contracted with a center for CATV must pay a feemonthly or a fee for every program in order to receive televisionprograms from the CATV. Accordingly, it is necessary to be able toprevent the poaching of pay television programs. There are somescrambling systems where the television signal is processed at thecenter so that the television signal can not be received through anordinary television set.

In a jamming method as one of the scrambling systems, an interferencewave or noise is inserted in a band of the CATV signal.

FIG. 13 shows a conventional jamming signal producing system. The systemis provided in a trunk line to subscribers.

A TV signal from a center is applied to a distributor 11a of the systemthrough a buffer amplifier 11. The signal is distributed and applied toindividual television sets of subscribers through combiners 10, 10a and10b, respectively.

The jamming signal producing system comprises a CPU 1 having a memory 5,a PLL (phase-locked loop) including a phase detector 2, a 1/N divider 2aand a VCO (voltage-controlled oscillator) 3, an A/D converter 4 and aD/A converter 6. The CPU 1 is operatively connected to the PLL 2, A/Dconverter 4, and D/A converter 6. The phase detector 2 is connected tothe VCO 3 through a switch S1 and to the A/D converter 4. The switch S1has two fixed contacts C1 and C2 operated by an output of the CPU 1. TheD/A converter 6 is connected to the VCO 3 through the contact C2 of theswitch S1. The VCO 3 is connected to switches S2, S2a and S2b through abuffer amplifier 7. The switches S2, S2a and S2b are closed by theoutput of the CPU 1 to connect VCO 3 to the low pass filters 9, 9a and9b, respectively.

To generate a jamming signal, the CPU 1 operates to close the contact C2of the switch S1. The data stored in the memory 5 is applied to the D/Aconverter 6 in which the data is converted into an analog signal. Theanalog signal is applied to the VCO 3 through the switch S1. The VCO 3oscillates, namely produces a jamming signal having a frequency based onthe data. The jamming signal is applied to the combiners 10, 10a and 10bthrough the low pass filters 9, 9a and 9b in accordance with theoperation of the switches S2, S2a and S2b. The jamming signal iscombined with the TV signal from the distributor 11a in the combiners10, 10a and 10b and the combined signals are fed to individualtelevision sets of the subscribers.

As shown in FIG. 14, in the combined signal, the jamming signal Fj isinserted a band including a video signal FV and an audio signal Fa. Thefrequency of the jamming signal stands from the video signal Fv by 250kHz.

If temperature or humidity around the jamming signal producing devicechanges, a drift occurs in the frequency of the jamming signal. If thedrift occurs, the frequency interval of 250 kHz is not maintained sothat a sufficient scramble effect is not obtained, or the jamming signalinterferes an adjacent channel. Thus, it is necessary to performcalibration to correct the drift.

In calibration operation, the CPU 1 operates to close the contact C1 ofthe switch S1. The output of the phase detector 2 is applied to the A/Dconverter 4 wherein the output signal is converted into a digitalsignal. The digital signal is applied to the CPU 1 and compared with thedata stored in the memory 5, so that an output signal in considerationof the drift is produced from the CPU 1 and applied to the D/A converter6. Then, the contact C2 closes, and hence the analog signal converted atthe D/A converter 6 is applied to the VCO 3 which oscillates the jammingsignal at a calibrated frequency. The calibration is continuouslyperformed at interval of 10 to 15 minutes.

In order to effectively scramble the television signal, it is necessaryto accurately control the amplitude, frequency and cycle of the jammingsignal.

It is recognized by experience that the amplitude of the jamming signalis selected to increase the jamming signal by 12 to 14 dB, the frequencyis larger than the video signal Fv by 250 kHz, and the cycle for jammingthe image is several tens msec.

In the system, since the drift is often calibrated, the jamming signalis not sufficiently inserted the TV signal, and it is impossible to scanthe channel to be scrambled at high speed so that the scramble effectcan not be ensured.

Further, the reduction of the lock-up time in the PLL circuit is notconsidered, which also aggravates the scramble effect.

The A/D and D/A converters provided in the system are expensive, causingthe increase of the manufacturing cost of the system.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a jamming signalproducing system which may scan a channel to be scrambled at a highspeed without calibration and is manufactured at a low cost.

According to the present invention, there is provided a system forproducing a jamming signal to be inserted in a band of CATV, comprising,a first jamming signal producing circuit including a control voltageproducing circuit for producing a control voltage corresponding to theband, and a first voltage-controlled oscillator for producing a firstjamming signal having a first jamming frequency dependent on the controlvoltage, voltage storing means for storing the control voltage, secondjamming signal producing means for producing a second jamming signal,and control means for controlling the production of the first and secondjamming signals.

In an aspect of the invention, the first jamming signal producingcircuit is a phase-locked loop comprising the first voltage-controlledoscillator, a programmable divider controlled by the control means forproducing a divided frequency corresponding to the band, a phasedetector for comparing the divided frequency with a reference frequencyand for producing a difference frequency, and a low pass filter forconverting the difference frequency to the control voltage, and thevoltage storing means includes capacitors and first switches operated bythe control means for controlling the storing of the control voltage inthe capacitors.

In another aspect, the second jamming signal producing means comprisessecond switches operated by the control means for controlling thedischarging of the control means for controlling the discharging of thecontrol voltage stored in the capacitors, and a secondvoltage-controlled oscillator for producing the second jamming signalhaving a second jamming frequency dependent on the discharged voltage.

Other objects and features of this invention will become understood fromthe following description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing a jamming signal producing systemaccording to the present invention;

FIG. 2 is a schematic diagram showing a second embodiment of the presentinvention;

FIG. 3 is a diagram showing a relationship between a first voltage Vt1and a time;

FIG. 4 is a diagram showing a relationship between a second voltage anda time;

FIG. 5 is a schematic diagram showing a third embodiment of the presentinvention;

FIG. 6 is a diagram showing jamming signals corresponding to respectivechannels;

FIG. 7 is a schematic diagram showing a fourth embodiment of the presentinvention;

FIG. 8 is a schematic diagram showing jamming signals oscillated by asingle oscillator;

FIG. 9 is a schematic diagram showing jamming signals oscillated by twooscillators;

FIG. 10 is a schematic diagram showing jamming signals for a weightingoperation;

FIG. 11 is a schematic diagram showing a fifth embodiment;

FIG. 12 is a schematic diagram showing a sixth embodiment;

FIG. 13 is a schematic diagram showing a conventional jamming signalproducing system; and

FIG. 14 is a diagram showing the jamming signal and audio/video signals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a jamming signal producing system of the presentinvention has a jamming signal producing circuit comprising a PLLincluding a first VCO 17 which produces an audio jamming signal for anaudio signal. The system further has a second VCO 33 which produces avideo jamming signal for a video signal.

The PLL comprises a 1/N divider 14, a programmable divider 26, a phasedetector 27, a low pass filter (LPF) 28, and the first VCO 17. Theoutput of the programmable divider 26 is connected to input of the phasedetector 27. The phase detector 27 compares the output of theprogrammable divider 26 with a reference frequency from a referencefrequency generator RF to produce a difference phase. The differencephase is converted to a DC voltage Vt1 by the LPF 28. The output of theVCO 17 is connected to a combiner 21 through an amplifier 19 and aswitch 20a. The VCO 33 is connected to the combiner 21 through anamplifier 34 and a switch 35a. The combiner 21 is connected to acombiner 23 through a low pass filter 22.

The system has a CPU 24 for controlling the operation of the system. TheCPU 24 is operatively connected to the programmable divider 26, theswitches 20a and 35a, and switches 29a to 29e and 30a to 30e. The CPU 24applies a control signal to the programmable divider 26 so that thephase detector 27 produces an output necessary for producing a controlvoltage Vt1 corresponding to a channel to be scrambled. Between theswitches 29a to 29e and 30a to 30e and the ground, capacitors 32a to 32eare connected to the respective switches. The capacitors 32a to 32echarge the voltage corresponding to the channels, for example Nos. 2, 3,4, 5 and 6, to be scrambled, respectively. The switches 30a to 30e areconnected to be connected to the VCO 33. The output of the LPF 28 isconnected to the capacitors 32a to 32e through the switches 29a to 29e.

A TV signal from a center is fed to a subscriber through the combiner23.

The operation of the system will be described hereinafter. In thesystem, the channels 2 to 6 are scrambled.

The control signal from the CPU 24 is applied to the programmabledivider 26 in accordance with a program, so that the divider produces anoutput having a frequency corresponding to a channel to be scrambled.The phase detector 27 compares the output with a reference frequencyfrom a reference frequency generator RF to obtain a difference phase.The difference phase is converted to the control voltage Vt1. Thevoltage Vt1 is applied to the first VCO 17. Thus, the first VCO 17produces an audio jamming signal having a frequency corresponding to achannel to be scrambled. The audio jamming signal is applied to thecombiner 21 through the amplifier 19 combiner and the switch 20a.

The voltage Vt1 is further applied to the switches 29a to 29e. Theoperation of each of the switches 29a

The voltager Vt1 is further applied to the switches 29a to 29e. Theoperation of each of the switches 29a to 29e is controlled by the outputof the CPU 24, corresponding to the scrambled channel. The switches 29ato 29e are sequentially turned on at regular intervals. Each switch isclosed for a period, for example of 10 msec. The capacitors 32a to 32eare charged with the voltage Vt1 corresponding to the respectivechannels to be scrambled.

Each of the capacitors 32a to 32e is sequentially discharged. Theswitches 30a to 30e are turned on in order of the channel in a cycle Tof 200 μsec by the output of the CPU 24. Thus, the voltage charged inthe capacitor is applied to the second VCO 33 as a control voltage Vt2.The second VCO 33 produces a video jamming signal for the video signalcorresponding to the selected channel to be scrambled. The jammingsignal is applied to the combiner 21, through the amplifier 34 and theswitch 35a.

The jamming signals for the sound and the image are applied to thecombiner 23 through the low pass filter 22. The TV signal is combinedwith the jamming signals to be scrambled.

If a drift of the frequency of the jamming signal from a VCO 17 occurs,the PLL operates to correct the drift. More particularly, if thefrequency of the jamming signal increases, the frequency of the divider26 increases. However, the output voltage of the Low Pass Filter 28decreases with the increment. Thus, the output frequency of the VCO 17is corrected to the desired frequency.

Referring to FIG. 2 showing the second embodiment, the same parts as thefirst embodiment are identified with the same reference numerals as FIG.1.

The system has a main microcomputer 16 and a sub microcomputer 16aconnected to the main microcomputer 16 through a bus line 25.

The PLL has the programmable divider 26 and the phase detector 27. Thecontrol signal of the main microcomputer 16 is applied to theprogrammable divider 26. The phase detector 27 is applied with areference frequency signal from the reference frequency generator RF anda divided frequency signal from the programmable divider 26. The outputof the phase detector 27 is applied to the first VCO 17 through the LPF28, and the output of the first VCO 17 is applied to the programmabledivider 26 through an amplifier 18. The output of the LPF 28 is furtherconnected to the analog switches 29a to 29e.

A gate circuit 20 connected to the VCO 17 through the amplifier 19 isoperated by the output of the main microcomputer 16. The mainmicrocomputer 16 is connected to a jamming channel set section 24a whichproduces a signal for indicating a channel to be scrambled. Themicrocomputer 16 has a memory (not shown) which stores data of jammingcarrier frequency of each channel.

The sub microcomputer 16a has a memory (not shown) which stores data ofinformation of each channel transmitted from the center. The submicrocomputer 16a is operatively connected to analog switches 30a to30e. Resistors 31a to 31e are connected between the analog switches 29ato 29e and the capacitors 32a to 32e.

A gate circuit 35 connected to the second VCO 33 through the amplifier34 is operated by the output of the sub microcomputer 16a.

The operation of the system will be described hereinafter with referenceto FIGS. 3 and 4.

As shown in FIG. 3, the voltage Vt1 is applied to the first VCO 17 atthe frequency of a cycle T1 of 50 msec for five channels 2 to 6 andhence a period t1 of each channel is 10 msec. When the voltage Vt1 forthe channel 2 is applied to the VCO 17, the switch 29a is closed,thereby charging the capacitor 32a with the voltage Vt1. Othercapacitors are similarly charged in sequence.

The voltage Vt2 is applied to the second VCO 33 at the frequency of acycle T2 (200 μsec) for a period t2 (50 μsec) for each channel. FIG. 4shows one cycle of the application of Vt2 for the channel 3 to channel 6during the application of VT1 for the channel 2.

The system is operated approximately in the same manner as the system ofthe first embodiment.

Namely, the phase detector 27 produces an output at frequencycorresponding to the difference between the frequency of the dividedoutput from the divider 26 and the reference frequency. The DC voltageVt1 from the LPF 28 is applied to the first VCO 17 which generates anaudio jamming signal at a frequency corresponding to the frequency andphase indicated by the control signal from the microcomputer 16. Thejamming signal is applied to the combiner 21 through the amplifier 19and the gate circuit 20.

When the DC voltage Vt1 for the channel 2 from the LPF 28 is applied tothe analog switch 29a, the analog switch 29a is closed in accordancewith the control signal of the main microcomputer 16 for a periods of 10msec. Thus, the capacitor 32a is charged with the voltage Vt1 of thechannel 2.

Similarly, when the voltages Vt1 corresponding to the respectivechannels 3 to 6 are applied to the analog switches, the switches 29b to29e are closed for 10 msec in order to charge the voltages in thecapacitors 32b to 32e, respectively.

The sub microcomputer 16a operates to turn on the analog switches 30a to30e in order for a period t2 in the cycle T2. When the sub microcomputer16a reads the data in the main microcomputer 16 through the bus line 25that the first VCO 17 scans the channel 2, the analog switches 30b to30e for the other channels are turned on and off at a high speed. Asshown in FIG. 6, the second VCO 33 scans the other channels, jumpingover the channel scrambled by the first VCO 17.

The voltage Vt1 charged in each of the capacitors 32a to 32e is appliedto the second VCO 33 as the voltage Vt2. The VCO 33 generates a videojamming signal which is applied to the combiner 21 through the amplifier32 and the gate circuit 35 for a period t2 of the channel 2.

The jamming signal of the audio signal is applied to the combiner 21 fora period of 10 msec, and the jamming signal of the video signal isapplied to the combiner 21 for a period of 50 μsec. The combined jammingsignal is applied to the combiner 23 through the low pass filter 22.

FIG. 5 shows the third embodiment. The system is provided with a thirdVCO 36 which produces a video jamming signal of the video signal. TheVCO 36 is connected to analog switches 30f to 30f which are in turnconnected to analog switches 30a to 30e in parallel. The submicrocomputer 16a is operatively connected to the analog switches 30f to30j. The third VCO 36 is connected to a combiner 39 through an amplifier37 and a gate circuit 38 operated by the sub microcomputer 16a. Thesecond VCO 33 is connected to the combiner 39 through the amplifier 34and the gate circuit 35. The combiner 39 is connected to the combiner21.

In the third embodiment, the jamming signal is applied at a shortercycle as shown by a dashed line of FIG. 6. Consequently, the jammingeffect of the video signal is enhanced.

Referring to FIG. 7 showing the fourth embodiment, the same parts as thefirst embodiment are identified with the same reference numerals as FIG.1.

The system has a first oscillator OSC1 and a second oscillator OSC2.

The first oscillator OSC1 comprises a control voltage generator 35, theprogrammable divider 26, phase detector 27 and LPF 28. The output of theLPF 28 is connected to the analog switches 29a to 29e through a bufferamplifier 42 having a small impedance. The analog switches 30a to 30eare connected to the VCO 17 through a buffer amplifier 43 having a largeimpedance. The output of the VCO 17 is applied to the combiner 23through a switch 20b and to the programmable divider 26 through the 1/Ndivider 14. The switch 20b is operatively connected to the CPU 24.

The second oscillator OSC2 has the same construction as the firstoscillator OSC1 and the output thereof is connected to the combiner 23,so that those oscillators are cooperated with each other for scrambling.For example, the first oscillator OSC1 is used for producing an audiojamming signal and the second oscillator OSC2 is for a video jammingsignal. To give another example, the first oscillator OSC1 is used forjamming signals for the early part of the channels 2 to 6 and the secondoscillator OSC2 is for the latter part of the channels.

In operation for channels 2 and 3 in the latter example, the switches29a and 30a are closed at the same time, so that the voltage Vt1 isapplied to the first VCO 17 to form a PLL, and the capacitor 32a ischarged with the voltage. The switch 20b is closed to apply the audiojamming signal for channel 2 to subscribers for the period of 10 msec,as shown in FIG. 8. Thereafter, switches 30a and 30b are alternatelyclosed at regular intervals, thereby applying video jamming signals.Then, the switches 29b and 30b are closed for 10 msec, thereby applyingthe audio jamming signal for channel 3. Such an operation is repeated tothe jamming signals for channels 2 and 3.

Similarly, the second oscillator OSC2 generates jamming signal forchannels 4 to 6. Daring the application of the audio jamming signal forsome of the channels (2CH and 3CH in FIG. 8), the other channel (4CH to6CH) are not supplied with jamming signals, as shown in FIG. 8.

FIG. 9 shows the operation of the former example. The first oscillatorOSC1 produces audio jamming signals for channels 2 to 6, and the secondoscillator OSC2 generates video jamming signals for the channels.Consequently, there is no blank periods as shown in FIG. 9.

Since the amplifier 43 has a large impedance, the charge stored in thecapacitor (32) is prevented from decreasing during the formation of thePLL. In addition, since the amplifier 42 has a small impedance, thecapacitor is quickly charged. Thus, the control voltage Vt1 is stored inthe capacitor without reducing.

It will be understood that the control voltage Vt1 does not changebetween the charging and the discharging of the capacitor, in spite ofthe offset of DC voltage inherent in the analog switch at ON/OFFthereof.

FIG. 10 shows an operation for weighting video jamming signals forparticular channels 2 and 3. To this end, switches 30a and 30b areclosed at a double frequency to produce two times video jamming signalsas large as the other channels. Thus, the video jamming signals forchannels 2 and 3 are weighted as shown in FIG. 10, thereby enhancing thescrambling of the channels.

FIG. 11 shows the fifth embodiment. The same parts as the previousembodiments are identified with the same reference numerals as FIGS. 1and 7.

The system has the first oscillator OSC1 and the second oscillator OSC2,and the control voltage generator 35, and the CPU 24 which are usedcommon to the first and second oscillators. A switch S having two fixedcontacts C3 and C4 is connected to the programmable divider 26 forselectively connecting the first oscillator OSC1 and the secondoscillator OSC2 to the programmable divider.

The first oscillator OSC1 comprises the first VCO 17 connected to thecontact C3 through the 1/N divider 14, analog switches 29a to 29econnected to the LPF 28 of the control voltage generator 35 through theamplifier 42, capacitors 32a to 32e, and analog switches 30a to 30econnected to the VCO 17 through the amplifier 43.

The second oscillator OSC2 comprises the second VCO 33 the output ofwhich is connected to the contact C4 through a 1/N divider 14a, analogswitches 29A to 29E connected to the LPF 28 through a buffer amplifier44, analog switches 30A to 30E connected to the VCO 33 through a bufferamplifier 45, and capacitors 32A to 32E connected between analogswitches.

Outputs of the CPU 24 are connected to the programmable divider 26,switches S, 20a, 35a, analog switched 29a to 29e, 30a to 30e, 29A to29E, 30A to 30E.

When the contact C3 is closed by the output of the CPU 24, the PLL isformed at the first oscillator OSC1. When the contact C4 is closed bythe output of the CPU, the PLL is formed in the second oscillator OSC2.

The system of FIG. 11 operates in a similar manner to the system of FIG.7 to produce jamming signals of FIGS. 8 and 9. However, the system ofFIG. 11 eliminates disadvantages in the system of FIG. 7 as describedhereinafter.

If the system of FIG. 7 is arranged in such that the first oscillatorOSC1 produces the audio jamming signal and the second oscillator OSC2produces the video jamming signal shown in FIG. 9, both the jammingsignals are formed by separate PLLs of respective oscillators. It isdifficult to manufacture both oscillators to have the samecharacteristic so that the control voltage generator 35 in the firstoscillator OSC1 may generate the same control voltage as the secondoscillator OSC2, and hence VCO 17 produces the same output frequency asthat of OSC2. Therefore, it is likely that scrambling is not effectivelyperformed.

To the contrary, in the system of FIG. 11, the single control voltagegenerator 35 is provided to

To the contrary, in the system of FIG. 11, the single control voltagegenerator 35 is provided to produce the same control voltage for theaudio jamming signal and the video jamming signal. Thus, both VCOs 17and 33 oscillate at the same frequency for the designated channel. Thus,effective scrambling can be performed.

Referring to FIG. 12 showing the sixth embodiment of the presentinvention, a memory E having a series of memory units Mo to M7 isprovided in stead of the capacitors 32a to 32e. Selection of thememories is performed by addressing the memories.

The PLL is completed with the write/read data buses of the memory E.

The control voltage Vt1 is converted to digital value by an A/Dconverter 40 and stored in a corresponding memory unit M in accordancewith the addressing of the CPU 24. When scrambling, the control voltageVt1 stored in the memory unit is read in accordance with the addressing,and converted to an analog value by a D/A converter 41, which is appliedto the VCO 17. The output of the VCO 17 is mixed with the output of theVCO 33 of the other oscillating unit (not shown) at the combiner 21 andfed to subscribers through an amplifier 46 and a gate 47.

In the system of the previous embodiments, the control voltage is storedin the capacitor. The stored voltage is liable to reduce because ofself-discharge. Therefore, it is undesirable to scramble for a longperiod.

In the system of FIG. 12, the control voltage Vt1 stored in the memoryis maintained at the set value while the capacitor in the previousembodiments. Accordingly, the period for scrambling can be set longerthan the previous systems, thereby effectively scrambling the CATVsignal.

In accordance with the present invention, the DC voltage is provided bya filter in the PLL. Consequently, the A/D and D/A converters providedin the conventional system are not used, so that the manufacturing costis reduced. Since the DC voltage is corrected in the PLL at everyjamming operation, the calibration of the drift of the frequency is notperformed, whereby a jamming signal having a constant frequency can beobtained.

While the presently preferred embodiments of the present invention havebeen shown and described, it is to be understood that these disclosuresare for the purpose of illustration and that various changes andmodifications may be made without departing from the scope of theinvention as set forth in the appended claims.

What is claimed is:
 1. A system for producing a jamming signal to beinserted in a band of CATV, comprising:a first jamming signal producingcircuit including a control voltage producing circuit for producing acontrol voltage corresponding to the band, and a firstvoltage-controlled oscillator for producing a first jamming signalhaving a first jamming frequency dependent on the control voltage;voltage storing means for storing the control voltage; second jammingsignal producing means for producing a second jamming signal; andcontrol means for controlling the production of the first and secondjamming signals.
 2. The system according to claim 1 wherein the firstjamming signal producing circuit comprises a phase-locked loop.
 3. Thesystem according to claim 1 wherein the voltage storing means includescapacitors and first switches operated by the control means forcontrolling the storing of the control voltage in the capacitors.
 4. Thesystem according to claim 1 wherein the second jamming signal producingmeans produces the second jamming signal from the control signalproduced by the control voltage producing circuit.
 5. The systemaccording to claim 1 wherein the second jamming signal producing meansis independent from the first jamming signal producing circuit.
 6. Thesystem according to claim 2 wherein the phase-locked loop comprises saidfirst voltage-controlled oscillator, a programmable divider controlledby the control means for producing a divided frequency corresponding tosaid band, a phase detector for comparing the divided frequency with areference frequency and for producing a difference phase, and a low passfilter for converting the difference phase to said control voltage. 7.The system according to claim 3 wherein the second jamming signalproducing means comprises second switches operated by the control meansfor controlling the discharging of the control voltage stored in thecapacitors, and a second voltage-controlled oscillator for producing thesecond jamming signal having a second jamming frequency dependent on thedischarged voltage.
 8. A system for producing a jamming signal to beinserted in a band of CATV, comprising:a jamming signal producingcircuit including a control voltage producing circuit for producing acontrol voltage corresponding to the band, and a voltage-controlledoscillator for producing a jamming signal having a jamming frequencydependent on the control voltage; voltage storing means for storing thecontrol voltage; and control means for controlling the production of thejamming signal.
 9. The system according to claim 8 wherein the voltagestoring means includes capacitors and switches operated by the controlmeans for controlling the storing of the control voltage in thecapacitors.
 10. The system according to claim 8 wherein the jammingsignal producing circuit comprises a phase-locked loop including saidvoltage-controlled oscillator, a programmable divider controlled by thecontrol means for producing a divided frequency corresponding to saidband, a phase detector for comparing the divided frequency with areference frequency and for producing a difference frequency, and a lowpass filter for converting the difference, and includes said switches.11. The system according to claim 8 wherein the jamming signal comprisesa first jamming signal for jamming a video signal and a second jammingsignal for jamming an audio signal.